Patient-Specific Surgical Guidance Tool and Method of Use

ABSTRACT

Presented is a preoperatively designed guidance tool for intraoperative use during bone or joint surgery wherein the guidance tool is specific to the anatomy of the patient being treated. The guidance tool comprises a body portion, a mating surface provided on the body portion for positioning the guidance tool on a corresponding registration surface of a patient&#39;s anatomy. The guidance tool further comprises at least one guide mechanism provided on the body portion for guiding a medical instrument at one or more preoperatively defined trajectories relative to a patient&#39;s anatomy. In the event of misalignment, the at least one guide mechanism is adjustable to alter the one or more preoperatively defined trajectories if necessary during intraoperative use. Also presented is a preoperative process for designing the guidance tool.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/917,713 filed on May 14, 2007 which is incorporatedherein by reference.

FIELD

The present invention pertains to a patient-specific alignment andguidance tool for a surgical procedure, and software associated withdesigning such a tool. The present invention also pertains to apre-operative process using the software for designing thepatient-specific alignment and guidance tool. The present invention alsopertains to an intraoperative process wherein the patient-specificalignment and guidance tool is used during a surgical procedure.

BACKGROUND

Bone and joint surgical procedures are well known in the art. To improveon conventional surgical techniques, imaging technologies and computersare increasingly being adopted and implemented by surgeons. The primarydrive in developing such technology is to reduce the overallinvasiveness of the procedure, while maintaining or increasing theoverall accuracy.

Computer assisted surgery (CAS) is now quite common for a range ofsurgical procedures. Surgeons are able to use computer trackingtechnology to visually map a patient's anatomy both before and duringthe surgical procedure. CAS also provides increased precision intargeting a particular site for correction. These techniques are alsovery useful for determining the optimal size and location of prostheticimplants.

Publications in the area of CAS have shown increased accuracy and/orprecision during the intraoperative procedure; however, these systemsadd not only higher costs to the surgery, but also increased surgicaltime. The necessary technical equipment related to conventionalcomputer-assisted systems makes additional surgeon and operating room(OR) team training necessary. Additionally, not all hospitals haveaccess to CAS techniques. Furthermore, the accuracy benefits of CAS arenot necessarily reflected in the patient outcomes, as there issignificant variability amongst surgeons in their ability to performprecise surgical operations.

Also known in the art is the use of templates for surgical procedures.Templates may be prepared using medical imaging techniques and they canenhance the accuracy of the procedure in the operating room. However, adrawback of such templates is that the alignment provided cannot beverified or adjusted intraoperatively.

In view of the foregoing disadvantages, it would be beneficial toimplement a procedure that combines the accuracy benefits of CAS withthe precision, repeatability, low cost and ease of use benefitsassociated with surgical alignment tools.

SUMMARY

According to one aspect, provided is a guidance tool for intraoperativeuse during bone or joint surgery wherein said guidance tool is specificto the anatomy of the patient being treated, said guidance toolcomprising a guide component for guiding a medical instrument at one ormore predetermined trajectories relative to a patient's anatomy, and aregistration component attached to said guide component for positioningsaid guidance tool on a patient's anatomy, wherein said guide componentcan be adjusted to alter the one or more predetermined trajectories ifnecessary during intraoperative use.

In one embodiment, the guidance tool further comprises a verificationcomponent for mechanically verifying correct position of the guidancetool on the patient's anatomy.

According to a further aspect, provided is a preoperative process fordesigning a guidance tool for intraoperative use during bone or jointsurgery wherein said guidance tool is specific to the anatomy of thepatient being treated, said process comprising creating a 3-D surfacemodel of the patient's anatomy, using the 3-D surface model of thepatient's anatomy to preoperatively determine a trajectory for pinplacement, designing a virtual guidance tool for registering against thepatient's anatomy, said virtual guidance tool providing a guidecomponent for guiding pin placement at the predetermined trajectoryrelative to the patient's anatomy, and a registration component attachedto said guide component for positioning said guidance tool on thepatient's anatomy, preoperatively assessing the fit, size and/or designof the guidance tool on the patient anatomy using the 3-D surface modelof the patient's anatomy, and adjusting the design of the guidance toolas necessary to achieve a correct fit.

According to another aspect, provided is a method of facilitating boneor joint surgery using a preoperatively designed patient-specificguidance tool, said method comprising the steps of placing the guidancetool directly upon the patient's anatomy for which it has been designed,said guidance tool providing a guide component establishing apredetermined trajectory for pin placement, assessing the predeterminedtrajectory governed by the guide component of the guidance tool,determining whether adjustments to the guide component are necessary forachieving an optimal placement of said pin, and adjusting, if necessary,the guide component of said guidance tool to achieve an alternatetrajectory for optimal placement of said pin.

According to a further aspect, provided is a method of verifying correctplacement of a preoperatively designed patient-specific guidance tool,comprising intraoperatively placing the guidance tool directly upon thepatient's anatomy for which it has been designed, the guidance toolhaving a guide component establishing a predetermined trajectory for pinplacement and a verification component for mechanically verifyingcorrect position of the guidance tool on the patient's anatomy, andusing the verification component to mechanically verify correct positionof the guidance tool on the patient's anatomy.

According to an aspect, provided is a preoperatively designed guidancetool for intraoperative use during bone or joint surgery wherein saidguidance tool is specific to the anatomy of the patient being treated,said guidance tool comprising:

a body portion;

a mating surface provided on said body portion, said mating surface forpositioning said guidance tool on a corresponding registration surfaceof a patient's anatomy; and

at least one guide mechanism provided on said body portion, said atleast one guide mechanism for guiding at least one medical instrument atone or more preoperatively defined trajectories relative to a patient'sanatomy;

wherein said at least one guide mechanism is adjustable to alter the oneor more preoperatively defined trajectories if necessary duringintraoperative use.

In some embodiments, said body portion comprises a guide component, anda registration component, each of said at least one guide mechanism andsaid mating surface being provided on at least one of said guidecomponent and said registration component.

In some embodiments, said body portion further comprises a stabilitycomponent.

In some embodiments, said mating surface on said body portion isdiscontinuous having regard to the patient's anatomy.

In some embodiments, said mating surface comprises a profile that iscomplementary to characteristic anatomical landmarks found on saidregistration surface of a patient's anatomy.

In some embodiments, the guide tool further comprises a removableverification tool for mechanically verifying the correct position of theguidance tool on the patient's anatomy, having regard to preoperativelyselected anatomical landmarks.

In some embodiments, said verification tool is configured for insertioninto said guide mechanism.

In some embodiments, said verification tool comprises at least onegraduated section to determine misalignment of the guidance toolrelative to said one or more preoperatively defined trajectories.

In some embodiments, said verification tool comprises two graduatedsection for providing misalignment values relative to the patient'sanatomy, a first graduated section providing misalignment values in alongitudinal direction, and a second graduated section providingmisalignment values in a radial direction.

In some embodiments, said at least one graduated section is a slidingruler is provided with graduations for measuring said misalignment.

In some embodiments, the guidance tool further comprises locking keysprovided on said body portion for aligning said verification tool in apreoperatively defined position for comparison with previously selectedanatomical landmarks.

In some embodiments, said guide mechanism comprises a guide channel andcorresponding instrument sleeve for placement within said guide channel,said instrument sleeve for guiding said medical instrument at said oneor more preoperatively defined trajectories relative to a patient'sanatomy.

In some embodiments, said guide mechanism comprises a realignment sleevefor use in place of said instrument sleeve, said realignment sleeveproviding a realignment of the instrument trajectory where theestablished trajectory requires adjustment.

In some embodiments, said realignment sleeve is configured with atranslational offset relative to the central axis of the sleeve.

In some embodiments, said realignment sleeve is configured with anangled offset relative to the central axis of the sleeve.

In some embodiments, said realignment sleeve is configured with acombined translational offset and angled offset relative to the centralaxis of the sleeve.

In some embodiments, the translational offsets are from about 0 to about5 mm from the central axis.

In some embodiments, the translational offsets are provided inincrements ranging from about 0.01 mm to about 1 mm.

In some embodiments, the angled offset is from about 0 to about 5°relative to the central axis.

In some embodiments, the angled offsets are provided in incrementsranging from about 0.05° to about 1°.

According to another aspect, provided is a preoperative process fordesigning at least one guidance tool for intraoperative use during boneor joint surgery wherein said at least one guidance tool is specific tothe anatomy of the patient being treated, said process comprising

creating a 3-D surface model of the patient's anatomy;

using the 3-D surface model of the patient's anatomy to preoperativelydetermine a trajectory of at least one medical instrument during bone orjoint manipulation;

designing at least one guidance tool for registering against thepatient's anatomy, said at least one guidance tool providing a guidemechanism and a mating surface, said guide mechanism for guiding said atleast one medical instrument during said bone or joint manipulation atthe preoperatively defined trajectory relative to a patient's anatomy,said mating surface providing positional registration of said at leastone guidance tool on the patient's anatomy; and

preoperatively assessing the fit, size and design of the at least oneguidance tool on the patient anatomy using the 3-D surface model of thepatient's anatomy, and adjusting the design of the at least one guidancetool as necessary to achieve a correct fit.

In some embodiments, said 3-D surface model of the patient's anatomy iscreated by one or more of CT, MRI, X-ray, and ultrasound.

In some embodiments, said assessment of fit involves a quantitativefigure of merit determination based on an examination of characteristiclandmarks on said mating surface relative to said patient's anatomy.

In some embodiments, the preoperative process further comprises theidentification of at least one characteristic anatomical landmark to beused in connection with a removable verification tool for mechanicallyverifying correct position of the at least one guidance tool on thepatient's anatomy, said verification tool being configured forattachment to the at least one guidance tool.

In some embodiments, multiple guidance tools are designed forintraoperative use for a particular procedure.

According to a further aspect, provided is a method of facilitating boneor joint surgery using a preoperatively designed patient-specificguidance tool, said method comprising the steps of:

placing the guidance tool directly upon the patient's anatomy for whichit has been designed, said guidance tool providing a mating surface anda guide mechanism, said guide mechanism for guiding at least one medicalinstrument during bone or joint manipulation at a preoperatively definedtrajectory relative to a patient's anatomy, said mating surfaceproviding positional registration of said guidance tool on saidpatient's anatomy;

assessing the position of the guidance tool upon the patient's anatomy;

assessing the preoperatively defined trajectory governed by the guidemechanism of the guidance tool;

determining whether adjustments to the guide mechanism are necessary forachieving an optimal trajectory of said at least one medical instrument;and

adjusting, if necessary, the guide mechanism of said guidance tool toachieve an alternate trajectory for said at least one medicalinstrument.

In some embodiments, a verification tool, fitted to said guide mechanismof the guidance tool, is used to assess the position and orientation ofthe guidance tool having regard to the preoperatively definedtrajectory.

In some embodiments, said verification tool comprises at least onegraduated section to determine misalignment of the guidance toolrelative to said one or more preoperatively defined trajectories.

In some embodiments, said verification tool comprises two graduatedsections for providing misalignment values relative to the patient'sanatomy, a graduated section providing misalignment values in alongitudinal direction, and a second graduated section providingmisalignment values in a radial direction.

In some embodiments, said at least one graduated section is a slidingruler provided with graduations for measuring said misalignment.

In some embodiments, said guide mechanism comprises a guide channel andan instrument sleeve, and wherein a correction to realign saidtrajectory is provided by a realignment instrument sleeve used in placeof said instrument sleeve to achieve the corrected trajectory.

In some embodiments, said guidance tool is retained in place on thepatient's anatomy manually by hand.

In some embodiments, said guidance tool is retained in place on thepatient's anatomy using suitable fasteners selected from the groupconsisting of pins, screws, straps, clamps, zip-ties and elasticfasteners.

In some embodiments, said verification tool is further used to visualizeand assess the preoperatively defined trajectory governed by the guidemechanism, wherein said verification tool is rotatable relative to saidguidance tool.

According to a still further aspect, provided is a method of realigninga trajectory in a preoperatively designed patient-specific guidancetool, comprising:

intraoperatively placing the guidance tool directly upon the patient'sanatomy for which it has been designed, the guidance tool having a guidemechanism establishing a preoperatively defined trajectory for at leastone medical instrument and a verification tool for mechanicallyverifying correct position of the guidance tool on the patient'sanatomy; and

using the verification tool to mechanically verify correct position ofthe guidance tool on the patient's anatomy.

In some embodiments, upon detection of a misalignment, said verificationtool is used to quantify the misalignment using at least one graduatedsection provided on said verification tool.

In some embodiments, upon detection of a misalignment, said verificationtool is used to quantify the misalignment using at least two graduatedsections provided on said verification tool.

In some embodiments, at least one of a reference chart, table orrealignment calculator is used to determine a correct realignmenttrajectory based on the registered misalignment, so as to return to thepreoperatively defined trajectory.

In some embodiments, said reference chart and/or table arepreoperatively generated based on preoperative planning having regard tothe geometry of the guidance tool and the patient's specific anatomy.

In some embodiments, said guide mechanism is adjusted to accord with thecalculated realignment trajectory.

According to yet another aspect, provided is a method of verifyingcorrect placement of a preoperatively designed patient-specific guidancetool, comprising:

intraoperatively placing the guidance tool directly upon the patient'sanatomy for which it has been designed, the guidance tool having a guidemechanism establishing a preoperatively defined trajectory for at leastone medical instrument and a verification tool for mechanicallyverifying correct position of the guidance tool on the patient'sanatomy; and

using the verification tool to mechanically verify correct position ofthe guidance tool on the patient's anatomy.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the attached Figures, wherein:

FIG. 1 is a skeletal representation of a human hip joint;

FIG. 2 is a skeletal representation of a human femur;

FIG. 3 is a 3-D computer iso-surface model representing bone and/or softtissue of the proximal femur and acetabulum;

FIGS. 4a and 4b show the establishment of a drilling trajectory for thecentral pin;

FIG. 5 is a 3-D computer iso-surface model representing bone and/or softtissue of the proximal femur and acetabulum, wherein the virtual femoraland acetabular components are fitted to the virtual model;

FIG. 6 is a 3-D computer iso-surface model representing bone and/or softtissue of the proximal femur, wherein the guidance tool is fitted to thevirtual model;

FIGS. 7a to 7c are perspective views of the femur with the guidance toolin position, wherein FIGS. 7b and 7c show a verification tool used toverify correct positioning of the guidance tool on the femur;

FIG. 7d is a perspective view of an embodiment of the verification toolused with the guidance tool to verify correct positioning of theguidance tool on the femur;

FIGS. 8a and 8b are perspective views of an embodiment of theverification tool in which it is used as a visual aid in verifying theplanned trajectory;

FIGS. 9a to 9d show a 3-D computer iso-surface model representing boneand/or soft tissue of the proximal femur, the series of illustrationsshowing a determined pin trajectory, a registration surface and aguidance tool comprising these features;

FIG. 10 is a perspective view of the femur, shown intraoperatively, withthe guidance tool in position;

FIGS. 11a and 11b are perspective views of the femur with the guidancetool in position, wherein the guidance tool is readied for guiding thecentral pin into the femoral head;

FIG. 12a illustrates a misalignment of the drilling trajectory.

FIGS. 12b and 12c are perspective views of alternate drilling sleevesfor use with the guidance tool wherein the guide holes are offset fromthe central axis;

FIG. 13 is a perspective view of the femur with the guidance tool inposition, wherein the central pin is being inserted into the drillsleeve for threaded insertion into the femoral head;

FIG. 14 is an schematic representation of how a guidance tool cansimilarly be used in mosaicplasty;

FIGS. 15a and 15b provide an illustration of how a guidance tool can beused in mosaicplasty of the knee;

FIG. 16 is an illustration of how a guidance tool can similarly be usedin distal radius osteotomy;

FIG. 17 is an illustration of how a guidance tool can similarly be usedin total knee arthroplasty;

FIG. 18 is an illustration of how a guidance tool can similarly be usedin total ankle arthroplasty;

FIG. 19a through 19c show an alternate guidance tool for total anklearthroplasty; and

FIG. 20 is a diagram representing the general steps in the preoperativeplanning stage in which a patient-specific guidance tool is createdbased on a patient's characteristic anatomy.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following discussion presents an embodiment wherein the patientspecific guidance tool and associated methodology/procedure forconstruction and implementation are presented largely within theframework of hip resurfacing. One will appreciate, however that theguidance tool and associated methodology/procedure may be implemented ina range of bone and joint surgical applications. For example, thefollowing discussion may find application for use in, but not limitedto, ankle, knee and shoulder surgery, spine fusion, craniomaxillofacialsurgery, osteotomies, fracture treatment and fixation, scoliosis, wristsurgery, and mosaicplasty. Thus, the following description is intendedas an exemplification of illustrative embodiments, and is not intendedto limit the description to the particular embodiments illustrated.

Unlike prior art technologies that incorporate intraoperativecomputer-assisted methods for pin-drilling guidance, the embodimentdiscussed below provides an individualized, patient-specific procedurethat begins prior to surgery. The method generally initiates with apre-operative planning stage in which a virtual 3-dimensional (3-D)iso-surface model of the patient's specific anatomy is created.Specifically, the anatomy of the patient undergoes preoperative medicalimaging by one or more of computed tomography (CT), magnetic resonanceimaging (MRI), ultrasound, X-ray, etc. With this data, a 3-D computeriso-surface model representing bone and/or soft tissue of the patient'sanatomy is produced, upon which the location and/or trajectory fordrilling, cutting, reaming, resurfacing, and/or modifying of bone and/orother tissue, for a central pin, guide pin or other surgical implement,can be preoperatively determined. The specific patient anatomy and thepreoperatively determined trajectories are then factored into themanufacture of a patient-specific guidance tool to assistintraoperatively. The preoperative design phase preferably includes avirtual assessment of quality of fit, and further enables the design andmanufacture of a verification tool to verify position/orientation of theguidance tool relative to the preoperatively determined trajectory andthe patient's anatomy.

As mentioned above, the patient-specific guidance tool and associatedmethodology/procedure for construction and implementation (generallytermed “patient-specific procedure”) is herein initially discussedwithin the framework of hip resurfacing. Hip resurfacing is a relativelynew procedure that is seen as an attractive alternative to total hipreplacement (THR) for younger, more active patients. In hip resurfacingthe femoral head is not replaced, as is done in THR, but instead afemoral component is implanted on the femoral head. The femoralcomponent includes a metallic hemisphere that effectively replaces thearticular cartilage covering the femoral head, and forms the bearingsurface of the femoral component, and one of the articulating surfacesof the hip joint. The femoral component also includes a central pin thatis inserted into the femoral head/neck, which maintains the correctposition of the femoral component. The hemisphere subsequently mateswith an acetabular component that is positioned in and provides a liningin the acetabulum of the pelvic bone, and provides the otherarticulating surface of the hip joint.

The most common cause of early failure of a hip resurfacing procedure isfemoral neck fracture. While careful patient selection can reduce therisk of neck fractures, a further significant contributing factor is theorientation of the hemi-spherical femoral component. A non-optimalorientation of the femoral component can result in notching of femoralneck cortex, which increases the risk of fracture. Also, excessivetilting of the femoral component in varus may result in greater stresseswithin the bone, which adds to the risk of neck fractures. The size ofthe component may also play an important role in early clinical success.The appropriate size of the femoral component should be large enough toprevent impingement, but small enough to avoid large bone resection ofthe acetabulum.

Referring now to FIG. 1, a skeletal representation of a hip is indicatedgenerally at reference numeral 10. The hip joint 20 is a synovial jointcomprised of the rounded head 22 of the femur 24 and the cup-likeacetabulum 26 of the pelvis 28; the primary function of this joint beingto support the weight of the body in both static and dynamic postures.

The femur 24 is shown in greater detail in FIG. 2. As shown, the femur24 is divisible into a body 30, and two extremities. The upper extremity32 comprises the head 22, a neck 36, a greater trochanter 38 and alesser trochanter 40; while the lower extremity 42 largely comprises amedial condyle 44 and a lateral condyle 46. Between the two extremitiesis the body 30 (or shaft) which is generally cylindrical in form.

Referring now to FIG. 3, hip resurfacing carried out in accordance withthe herein described patient-specific procedure begins with apre-operative planning stage in which the 3-D iso-surface model of thepatient's specific hip joint anatomy is created. As shown, the 3-Diso-surface model 50 represents to a high degree of accuracy the boneand/or soft tissue of the patient's anatomy. Shown in FIG. 3 is avirtual representation of the proximal femur 52 and the acetabulum 54.

Using the virtual 3-D iso-surface model 50, the drilling trajectory fora central pin or guide pin is preoperatively determined. As the centralpin will form the reference or guide for all subsequent procedures, itis desirable to establish and achieve a precise central pin trajectory.To determine the trajectory, a Virtual Surgical System (VSS) softwarepackage is used, developed by iGo Technologies Inc. (Kingston, Canada).One will appreciate, however, that other virtual surgical planningprograms are applicable and may be substituted.

As shown in FIG. 4a , using the VSS software and the established 3-Diso-surface model 50 of the proximal femur 52, the anatomicalposition/orientation of the neck-shaft axis 56 relative to the femur 52is registered. With this alignment information, the position andorientation of the trajectory 58 for the central pin is established. Thefemoral component 60, which comprises an articulating or bearing surface62 and the central pin is then fitted to the femur 52 in the virtualspace, as shown in FIGS. 4b and 5. In addition, and quiteadvantageously, having regard to the alignment information, theestablished trajectory 58 and the established positioning/orientation ofthe femoral component, the hip joint can be assessed for possibleintraoperative complications, such as notching of the femoral neckcortex during reaming of the femoral head. Based on these assessments,alterations to the positioning/orientation of the femoral component canmore safely and accurately be made preoperatively.

With the drilling trajectory 58 determined, the 3-D iso-surface model 50is then used to establish an accurate sizing of the femoral andacetabular components, so as to ensure that the femoral component doesnot infringe the femoral neck, and that excessive resection of theacetabulum is not required. As shown in FIG. 5, the 3-D iso-surfacemodel 50 serves as a pre-operative virtual space to fit the femoralcomponent 60 and the acetabular component 64 to the individualpatient-specific anatomy.

Continuing with the preoperative planning stage, the 3-D iso-surfacemodel and the established drilling trajectory information are used todesign a patient-specific guidance tool 66 for accurately achieving theplanned drilling trajectory for the central pin of the femoralcomponent. A virtual representation of the patient-specific guidancetool 66 relative to the patient-specific 3-D iso-surface model 50 isshown in FIG. 6. In general, the guidance tool comprises at least oneguide component that serves as a drill guide, at least one registrationcomponent for providing a mating surface relative to the patient'sanatomy, and optionally, at least one stability component for addingstability to the tool when placed upon the patient's anatomy.

As used herein, the term “mating surface” refers to a 3-D surface whichis complementary to and mates with the 3-D surface (registrationsurface) of a selected portion of a patient's anatomy. A guidance toolas described herein includes at least one mating surface.

As used herein, the term “registration surface” refers to the 3-Dsurface of a selected portion of a patient's anatomy.

In the embodiment shown in FIG. 6, the guidance tool 66 comprises a bodyportion 68 that is placed directly upon the patient's anatomy. Althoughthe body portion 68 of the guidance tool 66 can assume a wide range ofconfigurations, depending on the implementation, the body portion 68 asshown in FIG. 6 can be described by way of three components. First,provided is a guide component 70 for establishing the planned centralpin trajectory 58. The guide component 70 is provided with a guidemechanism 72 comprising a guide channel 74 for later receipt of aremovable instrument (e.g. drill) sleeve following correct and verifiedpositioning of the guidance tool on the patient's anatomy. Second,provided is a registration component 76 spanning the femoral neck, theregistration component 76 comprising a mating surface 78 (more clearlyshown in FIG. 7) for accurate registration on the patient's anatomy.Third, optionally provided is a stability component 80 for additionalstability during use. The body portion 68 of the guidance tool, inparticular the guide component 70 may optionally include one or morelocking keys 82 for use with a verification tool, as discussed below.

As mentioned above, the guidance tool 66 may take on any number ofconfigurations, depending on the implementation. As such, in alternateconfigurations, the guidance tool 66 may be comprised of a combinationof the above noted components. For example, while the guide component 70is configured to serve as a drill guide, it may also be provided with amating surface 78 for additional registration on the patient's anatomy.Additionally, the registration component 76, while comprising the matingsurface 78 for registration on a patient's anatomy, may also compriseone or more guide channels 74 for receiving drill sleeves. As will beappreciated, the patient-specific guidance tool 66 is designed forintraoperative use, whereby the guidance tool 66 is fitted to thepatient's anatomy. As indicated earlier, while the present embodiment isbeing presented within the framework of hip-resurfacing, the guidancetool and associated methodology/procedure for construction andimplementation may be suitably applied to a range of surgicalprocedures, such as, but not limited to ankle, knee and shouldersurgery, spine fusion, craniomaxillofacial surgery, osteotomies,fracture treatment and fixation, scoliosis, wrist surgery, andmosaicplasty. In addition, for any one surgical procedure, particularlythose during which multiple manipulations are necessary, it may bepreferable to create and use a plurality of guidance tools. For example,a first guidance tool may facilitate a set of drilling trajectories,while a subsequently used second guidance tool may facilitate shaping.Using a plurality of tools will assist in keeping the size of the toolto a minimum, and should also serve to avoid an unnecessarilycomplicated design.

In certain circumstances, the registration component 70 of the guidancetool 66 may comprise a discontinuous mating surface 78, particularly ifa portion of the imaging data is not suitable for rendering the surfacedetail of the patient's anatomy. In other instances, the registrationsurface 84 (see FIG. 6) of the patient's anatomy may not be suitable forincorporation into the registration component 76 of the guidance tool66. In a guidance tool 66 comprising such a discontinuous mating surface78, certain sections of the mating surface 78 may be voided in a mannerthat prevents interference with the patient's anatomy. For example, thediscontinuous portions may be sufficiently recessed to clear thepatient's anatomy.

In designing the patient specific guidance tool 66, a wide range offactors may be considered, so as to ensure the development of a specificcustomized product. For example, the guidance tool may be designedhaving regard to a chosen or preferred surgical approach or technique.For example, as one skilled in the art will appreciate, in hipresurfacing, the anterior-lateral approach is generally preferred topreserve a better blood supply to the femoral head, while the posteriorapproach serves to retain the capsular. The choice of anterior-lateralvs. posterior approach will have an overall impact upon the final designof the tool. Customization of the guidance tool based on a single chosensurgical approach has the advantage of simplifying the final design, asmultiple surgical approaches need not be engineered into a single tool.As such, the patient-specific guidance tool need not be unnecessarilylarge, and can therefore be appropriately sized to reduce theinvasiveness of the surgical procedure.

The guidance tool 66 is designed having regard to the specificregistration surface 84 of the patient's anatomy, namely characteristicanatomical landmarks, bony structures and/or soft tissue (e.g., tendons,ligaments, etc.). For clarity, anatomical landmarks, bony structures andsoft tissue are herein collectively termed “characteristic landmarks”.The characteristic landmarks of the registration surface 84 are used indesigning the mating surface 78 of the guidance tool 66, so as toregister it in a specific position/orientation necessary to attain thedesired drilling trajectory 58 determined during the pre-operativeplanning stage. The chosen surgical approach (e.g., anterior-lateral vs.posterior) may also have a bearing here, as certain characteristiclandmarks may be exposed depending on the selected approach. As will bediscussed below, the characteristic landmarks may also be usedintraoperatively to verify the position of the guidance tool 66 whenaffixed to the anatomy. Any combination of characteristic landmarks(anatomical landmarks, bony structures, soft tissue, etc.) may be usedfor registering and/or verifying correct positioning of the tool on thepatient's anatomy. This information is taken into consideration duringthe design of the guidance tool.

To verify the position/orientation of the guidance tool 66 on thepatient's anatomy, and to verify that the correct preoperatively definetrajectory is attained, the guidance tool 66 may be provided with aremovable mechanical verification tool 86, as shown in FIGS. 7b through7d . With the guidance tool 66 placed onto the patient's anatomy asshown in FIG. 7a , the verification tool 86 is configured to beremovably attached to the guidance tool 66 by inserting the registrationpin 88 of the verification tool 86 into the guide channel 74 of theguide component 70, as shown in FIG. 7b . The pointer 90 of theverification tool 86 can then be used, as shown in FIG. 7c , to locateone or more characteristic landmarks 92 that were preoperativelyidentified, so as to determine whether or not a proper positioning ofthe guidance tool has been established. To ensure accurate radialplacement of the verification tool 86 relative to the longitudinal axisof the planned central pin trajectory 58, one or more locking keys 82are provided on the guide component 70 to lock the verification tool inone or more predefined orientations. As shown in FIG. 7d , theverification tool 86 is provided with a recess 94 on the stem 96 forcooperating with the locking key 82 of the guide component 70, therebylocking the verification tool 86 in a fixed position. As will beappreciated, while a single characteristic landmark 92 may be used forposition verification, the procedure may use a plurality ofcharacteristic landmarks. As such, the guide component 70 may beconfigured with multiple locking keys 82, each corresponding to aparticular landmark.

The verification tool 86 is used to indicate whether or not the guidancetool 66 is correctly positioned on the patient's anatomy, and whether ornot the guidance tool establishes the preoperatively defined trajectory58. In the event of a misalignment, the user is able to quantifiablygauge the extent of misalignment using one or more graduated sectionsprovided on the verification tool 86. The graduated sections may includea scale such as a vernier or linear scale so as to allow measurement ofthe position of the guidance tool. For example, a graduated section mayinclude a sliding ruler. In the embodiment shown in FIG. 7d ,measurements along the longitudinal axis (x) of the planned central pintrajectory 58 are provided by a first sliding ruler 98, while radialmeasurements (y) are provided by a second sliding ruler 100 provided onthe pointer 90. Graduations 102 (e.g., mm) are provided on each ruler soas to provide a numerical indication of how much and in which directionthe guide is misplaced from the planned fitting position. Theverification tool may provide further indicators to characterize thenature of the misalignment. For example, it may be useful to obtain anangular misalignment value, that is the angle between the anatomicallandmark and the preoperatively defined position of the verificationtool. As such, a protractor-like scaling on the verification tool and/orguidance tool would be provided to facilitate this type of measurement.

Misalignments of the guidance tool may arise due to a variety ofreasons. For example, they may arise due to an error during thepreoperative planning stage. Misalignments may also arise due tophysiological changes in the patient, for example the formation ofosteophytes on the registration surface. To realign the guidance tool,removal of the osteophytes may be all that is required. In otherinstances, the misalignment may require the use of a realignment sleeve,as discussed in greater detail below. To assist the use in determiningthe necessary course of action to realign the guidance tool, the user ofthe guidance tool 66 is preferably provided with a reference chart,table or realignment calculator. The reference chart, table orrealignment calculator allows the user to determine an appropriaterealignment trajectory based on the characteristics of the misalignmentobtained from the verification tool, such that the preoperativelydefined trajectory can be attained.

The following demonstrates an exemplary use of the verification tool.For each anatomical landmark l; chosen, a pair of values (x,y) ispreoperatively determined, which describe the target location for thepointer of the verification tool. Intraoperatively during theverification of the guidance tool position, the verification tool isinserted into the guide channel and the values (x_m, y_m) are measuredfor the preoperatively determined anatomical landmark l_(i). If theguidance tool is correctly placed in the preoperative defined positionfor all landmarks the measured values (x_m, y_m) would be identical tothe preoperative determined values (x,y).

In case of misalignment, for one or more landmarks l_(i) a deviationbetween intraoperative measured values and preoperative determinedtarget values will occur. The characteristics of the misalignment,namely the direction and amount of misalignment of the landmarks can beused to identify a) position of registration-error, of b) plan ofcorrection possibility (realignment). Based on this information the usercan attempt an initial correction by, for example, removing osteophyte.If correction is not possible, the reference chart, table or realignmentcalculator is provided to navigate the user to the appropriatecorrection/realignment possibility.

The following example demonstrates the use of such a reference chart,table or realignment calculator using values obtained from theverification tool. For this exemplary implementation, two anatomicallandmarks l₁ and l₂ were chosen. The preoperative defined (x,y) valuesfor these landmarks are listed in Table 1.

TABLE 1 Preoperatively determined landmark coordinates x y l₁ 92 mm 21mm l₂ 57 mm 29 mm

With the verification tool, the preoperatively determined anatomicallandmarks are used to identify a misalignment. The direction and amountof the misalignment is determined.

Example 1

-   -   A set of exemplary intraoperative measured (x_m, y_m) values for        both landmarks are presented in Table 2.

TABLE 2 Intraoperatively measured landmark coordinates x_m y_m l₁ 92 mm23 mm l₂ 57 mm 31 mm

-   -   In this case, the deviation between the intraoperatively        measured y_m values and the preoperatively defined y values        indicate a misalignment in the direction of the frontal plane.        The amount of deviation, defined as

a=y_m−y

-   -   is used to identify the amount and type of displacement. With        respect to landmarks l₁ and l₂, the corresponding a₁ and a₂        values are calculated as follows:

a ₁ =y_m ₁ −y ₁=23−21=2

a ₂ =y_m ₁ −y ₁=31−29=2.

-   -   From this analysis, a translational misalignment is identified.        Having regard to the reference table (Table 3), the suggested        realignment is the use of a 2 mm translational offset in the        inferior direction.

TABLE 3 Translational offsets for corrections in Frontal Plane a A a 1−0.5 < a < 0.5 0.5 <= a < 1.5 −1.5 < a =< −0.5 . . . 2 −0.5 < a < 0.50.5 <= a < 1.5 −1.5 < a <= 0.5 No offset 1 mm offset 1 mm offsetsuperior inferior

-   -   -   Where, a=y_m−y            -   x,y=preoperative defined values            -   x_m, y_m=intraoperative measured values

Example 2

-   -   A second set of exemplary intraoperative measured (x_m, y_m)        values for both landmarks are presented in Table 4.

TABLE 4 Intraoperatively measured landmark coordinates x_m y_m l₁ 92 mm21 mm l₂ 57 mm 31 mm

-   -   In this example the deviation between the amount of displacement        between landmarks l₁ and l₂ identifies an angular displacement.        With respect to landmarks l₁ and l₂, the corresponding a₁ and a₂        values are calculated as follows:

a ₁ =y_m ₁ −y ₁=21−21=0

a ₂ =y_m ₂ −y ₂=31−29=2

-   -   The difference (d) between a₂ and a₁ is determined as

d=a ₂ −a ₁.

-   -   Accordingly, the difference is determined as follows:

d=a ₂ −a ₁=2−0=2

-   -   This angular displacement represents a 3° hyper-extension        misplacement of the guide, requiring the use of realignment        sleeve having a 3° flexion offset, as specified in the reference        table provided in Table 5.

TABLE 5 Angular offsets for correction in Frontal Plane d d d d −0.5 < d<= 0.0 0.0 < d <= 1.0 1.0 < d <= 1.5 1.5 < d <= 2.5 No offset 1° offset2° offset 3° offset flexion flexion flexion −1.0 =< d < 0.0 −1.5 <= d <−1.0 . . . 1° offset . . . extension

-   -   -   Where, a=y_m−y, d=a₂−a₁            -   x,y=preoperative defined values            -   x_m, y_m=intraoperative measured values

Based on the preoperative planning of the guidance tool 66, and theselection of the anatomical landmarks 92 for use with the verificationtool 86, the aforementioned reference chart or table can bepreoperatively calculated, such that cross-reference of thesemisalignment values provides the realignment necessary to achieve thepreoperatively defined trajectory 58. In a preferred embodiment, therealignment determination is provided by way of the realignmentcalculator, provided for example on a computer accessible within theoperative environment.

The realignment is provided by way of a realignment sleeve, as will bediscussed in greater detail below. With this approach to correctmisalignments, the user is not left with having to approximate thecorrection, but rather is provided with a directed calculated correctionbased on a specific measurements made with the verification tool.

Since the first sliding ruler 98 is generally configured to be parallelto the longitudinal axis of the central pin, the verification tool 86may also be used as a visual sighting aid to verify the chosen drillingtrajectory 58 and planned axis alignment, as shown in FIGS. 8a and 8b .By disengaging the verification tool 86 from the locking keys 82 androtating the verification tool 86 about the guidance tool 66, the useris able to visualise the trajectory 58 of the central pin with referenceto the plane defined by the first sliding ruler 98, or alternatively apin or guidewire inserted in place of the ruler. In some embodiments,the verification tool 86 may be configured to receive a separate pin orguide wire, also positioned parallel to the trajectory defined by theguidance tool. If it appears that a trajectory correction is necessary,suitable adjustments can be made as described in greater detail below.

The verification tool 86 may also be used intraoperatively to confirmthat the selected femoral component size is appropriate by measuring theradial distance between the planned central pin trajectory 58 and thesurface of the femoral neck. Alternatively, or in addition, this processcan also be used to confirm that the planned trajectory is substantiallycentered within the femoral neck.

The verification tool 86 as presented above is removable, thus allowingit to be reused. Alternatively, the verification tool 86 may beincorporated as a permanently fixed, integral component of the guidancetool 66. As such, there would be no need for the verification toollocking keys as described above. In such a case, the verification toolcould be machined to use the sliding rulers as discussed above, orconstructed as a single purpose non-adjustable pointer for identifying achosen characteristic landmark.

In the case of a removable verification tool 86, the tool may findfurther application once detached from the guidance tool. For example,the verification tool may be used to provide two or more referencespoints that are preoperatively determined to line up with certainanatomical landmarks, or to provide a gauge for assessing fit. Theverification tool, particularly where the tool is adjustable, can be setto a predefined setting that will enable verification that a particularpoint, for example on a prosthesis is correctly positioned at apreoperatively defined location relative to a preoperatively selectedanatomical landmark. The verification tool may find further applicationsin which a measuring device or gauge is useful in the operativeenvironment.

In instances where a plurality of guidance tools are used for aparticular procedure, a separate verification tool may be configured foreach guidance tool used. Alternatively, the verification tool may beused with the first guidance tool, with the positioning of subsequentguidance tools being verified relative to a common reference point asdetermined by the first guidance tool. For example, upon placement ofthe first guidance tool, the bone or cartilage may be marked using atleast one reference point. During placement of each subsequent guidancetool, to ensure proper alignment, each subsequent guidance tool isverified against this at least one reference point.

The general steps involved in pre-operative planning of thepatient-specific procedure for hip resurfacing are represented in FIGS.9a through 9d . As shown in FIG. 9a , based on medical imagingincorporating the patient's characteristic anatomical landmarks, thevirtual 3-D iso-surface model 50 of the patient's anatomy is firstcreated. The anatomical orientation/coordinates of the model aredetermined, the shaft/neck axis is registered, and the position andorientation of the femoral component is determined. FIG. 9a illustratesthe planned trajectory 58 of the central pin, based on the establishedshaft/neck axis. In FIG. 9b , a selected portion of the patient'sanatomy is analyzed to determine characteristic landmarks defining theregistration surface 84, as well as tool positioning verificationanatomical landmarks. As shown in FIG. 9c , an extracted surface image104 of the registration surface 84 is created. At this point in theprocess, the trajectory 58 of the central pin and the characteristics ofthe mating surface 78 have each been separately established. In FIG. 9d, the trajectory 58 of the central pin and the characteristics of themating surface 78 are combined into a single virtual guidance tool 66,wherein the planned trajectory 58 is governed by the guide component 70.As such, when used intraoperatively, the mating surface 78 of the tool66 is positioned on and registered with the patient's anatomy, asrepresented in FIG. 10, thus providing the planned trajectory 58 for thecentral pin.

The aforementioned task of designing the patient-specific guidance toolduring the preoperative planning stage is computer assisted using acustom software program. The software program is configured to design aspecific and customized guidance tool by taking into consideration thevarious factors discussed above. In particular, the software programprovides an interface enabling the user to analyze and identify thelocation of characteristic landmarks on the patient's anatomy, anddesign the guidance tool such that it positionally registers on thepatient's anatomy in a predefined orientation. In addition, the softwareprogram helps the user avoid structures not suitable for registration ofthe guidance tool. For example, certain soft and/or unstable bonystructures such as osteophytes are dismissed as viable registrationpoints.

The creation of the patient-specific guidance tool using theaforementioned software is generally a two step process. In the firststep, the software enables the user to plan the size, shape, position,and orientation of the guidance tool based on the patient's anatomy asdetermined in the initial medical imaging. In the second step, a 3-Drepresentation of the guidance tool is calculated comprising the variouscharacteristics specific to the patient's unique anatomy. The calculated3-D representation is then saved in a 3-D model format, such as astereolithographic format (e.g., standard tessellation language(STL-format)).

The first step of patient-specific guidance tool planning isaccomplished using available software packages such as Qt (TrolltechASA, Oslo, Norway; www.trolltech.com) and Coin3D (Systems in Motion AS,Oslo, Norway; www.coin3d.org). Qt allows high-performance cross-platformGUI (graphical user interface) development, while Coin3D is a high-level3D graphics toolkit for developing cross-platform real-time 3Dvisualization software. It will be appreciated, however, that oneskilled in the art could implement other suitable software programs inplace of those mentioned above.

Software suited for the first step of patient-specific guidance toolplanning preferably offers the following functions: a) establishment andloading of the 3-D surface model of a patient's anatomy; b)establishment and loading of surgical planning; c) planning of lockingkeys; and d) designing of patient-specific guidance tool.

The following paragraphs describe an example of patient-specificguidance tool planning as applied to hip resurfacing. Variations of thisprocedure will be apparent to one skilled in the art.

Based on preoperative medical imaging, the 3-D surface model of apatient's anatomy is loaded and displayed to the user. Next, thedetermined trajectory of the central pin is loaded and displayed as acylinder. The guidance tool is then designed, in this case as threedifferent parts. First, the registration component is oriented along thefemoral neck and contains the mating surface which mates theregistration surface of the patient's anatomy. Second, the stabilitycomponent is oriented around the lateral femoral neck, and contains amating surface that registers with at least a portion of theanterio-lateral or posterior-lateral neck. Third, the guide componentprovides the guide channel for the drill sleeves.

During patient-specific guidance tool planning, the user chooses thesize, position and orientation of the registration component. Tofacilitate the design process, a rectangular prism or cube representingthe registration component is displayed, allowing the user to modifywidth, height and length, as deemed necessary to achieve a proper sizingand fit. The user can further define the final position by altering theposition and rotation of the rectangular prism or cube around thecentral pin axis.

For planning of the stability component a virtual ring-segment isdisplayed, which is attached on the lateral side of the registrationcomponent. The user can modify the radius, angle, width and height ofthis ring segment and change its position and orientation, so as toachieve a proper sizing and fit.

For planning of the guide component a virtual cube is displayed, whichis attached on the medial side of the registration component. Similar tothe previous two components, the user can modify width, depth, andheight of this cube, so as to optimize the sizing and fit.

At this stage, the planning must also consider the verification tool andthe associated locking keys. It is during this planning that the userselects one or more anatomical landmarks on the femoral model that willalign with the verification tool. As described above, the alignment ofthe verification tool is governed by the locking keys. As such, based onthe location of the anatomical landmarks, and the configuration/size ofthe verification tool, the positioning of the locking keys on the guidecomponent is established.

In the second step wherein a 3-D representation of the guidance tool iscalculated, the mating surfaces for the registration component andstability component of the guidance tool are calculated. In oneexemplary methodology, the registration component and stabilitycomponent are represented each as a set of 6 planes, which are definedby a bounding box. To determine the registration surface of thepatient's anatomy, the surface representation of the bone is intersectedwith all six planes. For all resulting triangles the normal vectors areinverted and the mating surface is saved in a triangulated format.

For the final calculation of the registration and stability components,both virtual representations (cube and ring-segment) are saved asvolumetric objects (sets of tetrahedrons). An algorithm is then used tointersect each volumetric object along the calculated mating surface.During this calculation the distance of each tetrahedron with respect tothe mating surface is determined. Tetrahedrons which are intersected bythe surface are split in two sets of tetrahedrons, corresponding tothose below and above the surface. Finally all tetrahedrons above thesurface are combined into one volumetric object and this object isconverted into a surface model. The result is the surface representationof the registration and stability components of the guidance tool, whichcontains the mating surface complementary to the registration surface ofthe patient's specific anatomy.

For determination of the guide component, in particular the guidechannel, the surface of a cylinder is modeled, which is oriented alongthe determined pin trajectory. The outer shell of the guide componentand the locking keys are modeled as a rectangular prisms or cubes. Theguidance tool is determined by combining these models.

The mating surfaces which are integrated into the guidance tool allowintraoperative registration of the tool to the characteristic landmarksof the registration surface of the patient. The anatomical registrationsurfaces must provide sufficient features to allow a precise fit of thetool to the anatomy. Location and size of the mating surfaces aredefined by the shape, size, position and orientation of the guidancetool. To avoid intraoperative problems during fitting of the guidancetool to the anatomy, the mating surfaces can be evaluated for theirquality of fit during preoperative planning of the guidance tool.

Quality of fit may be assessed by examining registration features (i.e.,characteristic landmarks) of the registration surface and/or the matingsurface. Various publications in the area of registration duringcomputer-assisted surgery provide methods to evaluate registrationfeatures (Ma, B., Ellis, R. E. “A point selection algorithm based onspatial-stiffness analysis of rigid registration”, Computer-AidedSurgery, 2005 July; 10(4): 209-223; Simon, DA. “Fast and accurateshape-based registration”, PhD thesis, Carnegie Mellon University,Pittsburgh, Pa., December 1996). For clarity and by way of example only,the quality of fit of a mating surface can be determined in thefollowing way. Virtual copies of the mating surface are created. To eachof these copies a different error-transformation is applied and asurface-based registration algorithm is used to determine thetransformation between this modified mating surface and thecorresponding registration surface of the anatomy (Besl, P., McKay, N.“A method for registration of 3-D shapes.”, IEEE Trans. Pattern Anal.1992, 14(2): 239-256). After the calculated transformation is applied tothe modified mating surface copy, the distance between this matingsurface and the original mating surface is calculated and saved as anerror value. A chosen mating surface has sufficient registrationfeatures if the error values for all copies of the mating surface arebelow a predefined threshold. The error value is used to determine afigure of merit, which is indicative of quality of fit of the guidancetool to the patient's anatomy.

The figure of merit may also be representative of the level ofinvasiveness of the procedure. For example, the figure of merit mayrepresent an optimization of factors including tool fit and level ofinvasiveness. For example, reducing tool size results in a less invasiveprocedure but a smaller tool size may also reduce quality of fit. Aswill be appreciated, the figure of merit is provided by the software,whereas the manipulation of the size, orientation, etc. is thedesigner's choice based on what the designer is trying to achieve indesigning the tool.

An advantage of the patient-specific procedure and resultant guidancetool, which may be applied to any bone and joint surgical procedure, isthat the user may customize the tool, for example by selecting a desiredsurgical approach (e.g., an anterior-lateral approach versus a posteriorapproach) which best suits the users ability and/or the needs of thepatient.

Once the design is completed, the computer model data is saved in astereolithographic format (e.g., STL) suitable for subsequent guidancetool manufacture. Alternatively, other formats may be used as would beapparent to one skilled in the art.

In the present embodiment, the subsequent manufacturing of theindividual patient-specific guidance tool implements a rapid prototypingmachine, such as the Dimension SST 3-D printer provided by Stratasys,Inc (Eden Prairie, Minn.). One will appreciate, however, that othersuitable rapid prototyping machines may be used to manufacture theindividual patient-specific guidance tool. In such a process, rapidprototyping takes the design of the patient-specific guidance tool andtransforms it into virtual cross sections, followed by the creation ofeach cross section in physical space. The process continues through thevarious cross-sections until the physical model is finished. While avariety of metallic and non-metallic (e.g., polymers) materials may beused in rapid prototyping methods to make the guidance tool, the presentembodiment is fabricated from acrylonitrile butadiene styrene (ABS).Following manufacture, the guidance tool is readied for use, whichgenerally comprises sterilization and packaging using knownmethodologies.

In another embodiment, the patient-specific guidance tool may bemanufactured intraoperatively in real time using computer assisted toolgeneration.

In use during hip resurfacing, following conventional surgicalprocedures known in the art, the femur is dislocated from theacetabulum. The patient-specific guidance tool 66 is then placed ontothe desired part of the patient's anatomy as shown in FIG. 11a , havingregard to the positioning and fit of the mating surface of the guidancetool relative to the corresponding registration surface of the patient'sanatomy. While the guidance tool 66 may be fastened to the femur usingsuitable fasteners (e.g., pins, screws, etc), the guidance tool mayalternatively be held in place manually by hand, or with suitablestraps, clamps, zip-ties or elastic fasteners (e.g., elastic bands).Once registered to the bone, the verification tool 86 is used to verifythe correct positioning of the guidance tool. With correct positioningverified, a drill sleeve 106 is inserted into the guide channel 74, asshown in FIG. 11b . The sleeve 106 provides a guide hole 108 fordrilling and/or insertion of the central pin 110 at the planned centralpin trajectory 58. The sleeve 106 is preferably a surgical grade metal,including but not limited to surgical grade stainless steel andtitanium.

During intraoperative use, as discussed above, it may be necessary toalter the trajectory 58 of the central pin, either due to a misalignedguidance tool as identified by the verification tool, or due todecisions made in the operating room. An exemplary misalignment 112 of2° from a preoperatively defined trajectory 58 is shown in FIG. 12a .Should realignment or alteration of the trajectory of the central pin benecessary, realignment sleeves as shown in FIGS. 12b and 12c , can beused to drill holes that are offset by a known amount or are in adifferent orientation. For example, the realignment sleeve 114 shown inFIG. 12b has the guide hole 108 translationally offset from the centralaxis 116 of the sleeve, while the realignment sleeve 114 shown in FIG.12c has an angled guide hole 108. Guide hole 108 translational offsetsmay be anywhere from 0 to 5 mm from the central axis 116. For example,the offsets are provided in 1 mm increments (e.g., 1 mm, 2 mm, 3 mm, 4mm, 5 mm). Alternatively, the offsets are provided in 0.5 mm increments(e.g., 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm,4.5 mm, 5.0 mm). Still further alternatives with smaller increments maybe provided such as 0.1 mm increments, or 0.05 mm increments. Angledoffsets from the central axis 116 may be anywhere from 0 to 5°. Forexample, the angled offsets are provided in 1° increments (e.g. 1°, 2°,3°, 4°, 5°). Alternatively, the angled offsets are provided in 0.5°increments (e.g., 0.5°, 1.0°, 1.5°, 2.0°, 2.5°, 3.0°, 3.5°, 4.0°, 4.5°,5.0°). Still further alternatives with smaller increments may beprovided such as 0.1° increments, or 0.05° increments. Realignmentsleeves 114 with combined translational offsets and angled offsets mayalso be provided in which the aforementioned dimensional characteristicswould be applicable. Further, the drill sleeves 106 (and realignmentsleeves 114) may be configured with a stop or other suitable mechanismto precisely control the depth of the pin placement. To ensure the drillsleeve 106 (and realignment sleeves 114) remains in a fixed position,the sleeves may be indexed relative to the guidance tool. For example,the drill sleeve 106 (and realignment sleeve 114) may be provided with akeyed surface 118 that cooperates with a receiving channel (not shown)on the guidance tool 66. The keyed surface 118 ensures the placement ofthe drill sleeve 106, in particular the realignment sleeve 114 in thepreoperatively defined anatomical direction of the femur to ensure thecorrections are performed in the desired direction (i.e., varus/valgus,anterior/posterior, etc.).

To facilitate the above-noted correction, as discussed above withreference to the verification tool, a reference chart, table orrealignment calculator may be provided to assist in choosing the properrealignment sleeve to achieve the preoperatively defined trajectory. Thechoice of the correct realignment sleeve is based on the quantifiedmisalignment as determined using the sliding rulers of the verificationtool. The misalignment values are cross-referenced to the referencechart or table, or are inputted into the realignment calculator, and anappropriate realignment sleeve 114 is identified.

To further facilitate the above-described use, it may be advantageous toprovide the operating room personnel with a model of the patient'sanatomy. For example, the model may be provided as a paper printout, ascreen image on a monitor in the operating room, or preferably aphysical model. In certain circumstances, during intraoperative use, theuser may find that the guidance tool is not fitting correctly to thepatient's anatomy. For example, if a period of time passes between theinitial medical imaging and the intraoperative procedure, the patient'sanatomy could develop osteophytes that affect the overall fit of theguidance tool. Providing a model, in particular a physical model of thepatient's anatomy, could facilitate the troubleshooting process in theevent of guidance tool misalignment, if necessary.

To further enhance the adjustability of the guidance tool 66, the guidecomponent 70 may be adjustable relative to the registration component76. For example, in some embodiments, the guide component 70 may bedetachable or configured to be loosened from the registration component,whereby the guide component 70 can be subsequently retightened in aslightly realigned position. The capability of being realigned can beprovided, for example, by way of slotted or oversized holes throughwhich suitable fasteners would be used.

With the guidance tool 66 set in proper position/orientation, and thecorrect drill sleeve 106 or realignment sleeve 114 in place, the centralpin 110 can be drilled into the femoral head 22 using drillingmethodologies known in the art, as generally shown in FIG. 13.

Once the pin 110 is placed in the femur 24, the drill sleeve 106 (orrealignment sleeve 114) is removed over the pin and the tool 66 isremoved from the bone. The hip resurfacing procedure then continuesaccording to instructions and/or training provided by the implantmanufacturer.

As previously indicated, for the purposes of discussion, thepatient-specific procedure and resultant guidance tool described aboveis exemplified having regard to hip resurfacing. One skilled in the artwill appreciate that the above-noted patient-specific procedure andresultant guidance tool may find application in other surgicalprocedures, for example, knee, ankle and shoulder surgery, spine fusion,craniomaxillofacial surgery, osteotomies, fracture treatment andfixation, scoliosis, and wrist surgery.

FIG. 14 shows an embodiment in which the preoperativeprocedure/methodology and resultant guidance tool is used inmosaicplasty, a form of therapy designed to replace the articularcartilage of the highly loaded surface of a joint that has been damagedby trauma or arthritis. Damaged articular cartilage in weight-bearingareas, for example the knee, is not only painful for the patient, butalso limits the range of motion (ROM) and therefore has a great effecton the patient's quality of life. Surgical treatment is often the onlytreatment option, as the self-healing potential of articular cartilageis quite limited.

One treatment technique is the transplantation of multiple autologousosteochondral plugs from a donor region into the damaged region. Forlong term success of this procedure, the transplanted plugs shouldreconstruct the curvature of the articular surface. As such, manyparameters including size, height, position, orientation and rotation ofthe plugs, as well as number and pattern of the plugs must beconsidered.

As generally represented in FIG. 14, mosaicplasty traditionally involvestwo guidance tools 200, 202, the first 200 being used on the donor site204 to extract cartilage material, the second 202 being used on thereceiver (damaged) site 206 to ensure correct positioning of theextracted cartilage.

In the following exemplary embodiment of mosaicplasty of the knee, asdescribed above for hip resurfacing, the patient is subjected topreoperative medical imaging by one or more of CT, MRI, ultrasound,X-ray, etc. to reconstruct the knee's bony anatomy, cartilage thickness,and an outline of the cartilage defect. As shown in FIG. 15a , a virtual3-D iso-surface model 208 of the knee is computed, containing a bonemodel 210 and a transparent layer of cartilage 212. The damagedcartilage region 214 is next virtually restored by modifying thecartilage defects. The modified model is then used as the template forpreoperative planning of mosaicplasty, using the aforementionedsoftware. For each plug, the user can modify radius, height, positionand orientation in the donor and receiving (damaged) areas. A graphicaluser interface allows the user to plan and evaluate the quality of fitof the final virtual guidance tool.

To transfer the final plan into intraoperative use, thepatient-specific, sterilizable, plastic guidance tool 216 is formedusing rapid prototype technology. As shown in FIG. 15b , the guidancetool 216 includes a mating surface 218 complementary to the articularregistration surface 220 of the knee, enabling correct positioning ofthe tool 216 on the knee, thereby ensuring a precise transformation ofthe preoperative plan into the intraoperative surgical field. For eachplug, two instrument guides may be incorporated in the guidance tool.For example, on the donor side of the guidance tool, a donor guide 222is positioned to orient a plug cutting instrument with respect to thepreoperative planning. To ensure the planned height of the plug, it ispreferable to use a predefined height mark on the cutting instrument foralignment with the top edge of the guide. In a similar way, a guide 224on the receiving (damaged) side of the guidance tool 216 is provided tofacilitate navigation of the tools for preparation and transplantationof the plug into the damaged area. Both guides 222, 224 are preferablydesigned with instrument alignment marks to ensure that the rotation ofthe plug follows the planned curvature of the articular surface.

In some circumstances, the damaged anatomy may cover a sizable area. Forexample, it is not uncommon to have a cartilage defect of 2×3 cm on themedial site of the knee. To repair such a defect, a plurality ofautologous osteochondral plugs are generally required, the diameters ofwhich range from 4 mm to 8 mm.

For such a procedure, to facilitate intraoperative navigation, it may beadvantageous to use three separately formed patient-specific guidancetools. Each of the guidance tools serve to guide the harvesting andinsertion of 2-4 plugs. As previously mentioned, in instances wheremultiple guidance tools are used, a separate verification tool may beused for each guidance tool. Alternatively, the first guidance tool maybe configured for use with a verification tool, while positioning ofsubsequent guidance tools is verified using a set of reference points,common to each of the guidance tools. For example, as shown in FIG. 15b, the guidance tool 216 provides two reference points 226 that allow formarking of the cartilage upon placement of the first guidance tool.During subsequent steps, alignment of a second guidance tool (not shown)to these reference marks 226 ensures that the second guidance tool isaligned to the first guidance tool; the positioning of the secondguidance tool is verified. As each tool is placed upon the anatomy,alignment with the reference points ensures correct positioning of theguidance tool. With the guidance tool facilitating navigation, theprocedure itself may be conducted using conventional cartilage repairsystems, for example the COR cartilage repair system (Depuy Mitek Inc.,a Johnson and Johnson company, Warsaw, USA).

The preoperative planning and guidance tool provides a time-efficient,accurate, cost-effective and easy to use method of articular cartilagereconstruction, particularly in instances involving multiple autologousosteochondral plugs.

FIG. 16a through 16d provide an embodiment in which the preoperativeprocedure/methodology and resultant guidance tool is used in distalradius osteotomy. An osteotomy is a surgical procedure to realign a bonein order to change the biomechanics of a joint, especially to change theforce transmission through a joint. For distal radius osteotomy, a 3-Diso-surface model 300 of the radius 302 is created (from CT-data, and/orother suitable medical imaging) and the osteotomy is performed virtually(FIG. 16a ). A model of the plate 304 is virtually placed on theproximal and distal ends of the virtually corrected radius 302. Thescrew positions and orientations of the plate are saved and transformedonto the original radius (non-fractured). A patient-specific guidancetool 306 is created (see FIG. 16b ) in accordance with theprocedure/methodology described above, which provides guide mechanism308 for guided drilling of distal and proximal screw holes. During thesurgery, the distal end of the radius is surgically exposed and theguidance tool 306 is positioned, as shown in FIG. 16c . Followingverification using a verification tool, using drill sleeves (orrealignment sleeves) inserted into the guidance tool 306, the userdrills the screw holes into the radius 302, at the preoperativelyplanned trajectories 310 shown in FIG. 16d . The procedure may furtheruse a second guidance tool (not shown) to facilitate the shaping of thedistal surface using a Forstner drill bit, or other suitable tool. Theguidance tool 306 may be configured to facilitate both theposition/location and depth of milling in the shaping steps of theosteotomy. After the shaping is complete, the guidance tool 306 isremoved and the surgeon performs the osteotomy in the conventional way.The plate, for example a Synthes 3.5 mm locking compression plate 304,is fixed using the preoperatively planned screw holes.

FIGS. 17a and 17b provide a further embodiment in which the preoperativeprocedure/methodology and resultant guidance tool are used in total kneearthroplasty. In total knee arthroplasty, a 3-D iso-surface model 400 ofthe femur 402 and tibia 404 is created and the femoral distal and tibialresection is planned. A patient-specific guidance tool 406 for the femur402 is created (FIG. 17a ), which contains guide mechanisms for guidedplacement of two guidance pins 408. During the surgery the guidance tool406 is placed upon the patient's anatomy, positionally verified using averification tool and pins 408 are then placed into the femur 402. Afterremoving the guidance tool 406 the pins 408 are used to guide aresection block with respect to the preoperative planning of theresection plane. A similar guidance tool 410 is created for the tibia404 (FIG. 17b ), which contains the guide mechanism for guided placementof the pins 412. During surgery these guidance pins 412 allow placementof a resection block with respect to the planned tibial resection plane.As required, the guide mechanisms are adjusted in accordance with anycorrection required as indicated using the verification tool.

FIG. 18 provides a further embodiment in which the preoperativeprocedure/methodology and resultant guidance tool are used in totalankle arthroplasty. Total ankle arthroplasty has, in the past few years,become an accepted method in the treatment of end-stage ankle arthritis.Studies analyzing the long-term outcome of total ankle arthroplasty haveshown that initial implant alignment is a very important factor inavoiding intaoperative and post-operative complications. The literaturearound total ankle arthroplasty has shown a steep learning curve withadoption of this procedure. In particular, obtaining correct alignmentof the talar component is problematic since the surgical exposure allowsvisualization of only a small portion of the talus. This limited view,among other problems, makes it difficult to intraoperatively determinecorrect position of the initial surgical jig. Highlighting the need forproper component alignment in lower limb arthroplasty is the body ofresearch showing that mal-alignment and instability are the two mostimportant causes of early (i.e., less than five years) failure in totalankle arthroplasty.

In accordance with the preoperative procedure/methodology describedabove for hip-resurfacing, the patient is first subjected topreoperative planning in which a CT scan (or other suitable medicalimaging) is performed on the affected anatomy. As shown in FIG. 18 a,3-D virtual model 500 is produced from the medical imaging.

During pre-operative planning, the position, size and orientation of theprosthesis components 502 a, 502 b are determined, as shown in FIG. 18b. The virtual bone models 500 are then loaded, anatomical axes aredefined and virtual prosthesis components 502 a, 502 b are superimposedto the tibia and talus virtual models. At this stage, the user is ableto modify the position and orientation of each virtual prosthesis model.The characteristics of the final components are then saved.

Based on preoperative planning, the guidance tool 504 can then becreated using the rapid prototype technique, the guidance tool 504providing the mating surface 506 complementary to the distal tibia andtalus 508, which are accessible via the chosen surgical approach.

Intraoperatively, the total ankle arthroplasty begins with aconventional surgical approach. After the distal tibia and talus 508 areexposed, as shown in FIG. 18c , the guidance 506 tool is fitted to thecorresponding registration surface of the patient's anatomy, drillsleeves are inserted, or adjusted with realignment sleeves if necessary,and the guide pins 510 are drilled into position. With the guidance toolremoved, as shown in FIG. 18d , a conventional planar tibia resectionblock 512 can be fitted to the tibia 508. Resection can then beperformed. Position of the cutting guide may be confirmedfluoroscopically prior to cutting the bone.

While exemplified largely within the framework of drilling and pinplacement, the preoperative process described above may find applicationin the development of other guidance tools suited for use with othermedical instruments. Guidance tools may be preoperatively planned andmanufactured that are suitable for guiding instruments intended forcutting, reaming and resurfacing. Shown in FIG. 19a through 19c is analternate procedure and guidance tool 600 for Total Ankle Arthroplastyin which the guide mechanism comprises a guidance block 602 used toguide a saw blade, instead of setting guide pins as shown in FIG. 18c .In such a procedure, the preoperative planning of the guidance tool 600is similar to that discussed above, except that the preoperativelydefined trajectory is identified as a plane upon which the saw is to bedirected. When being used intraoperatively, the guidance tool 600 isfitted to the patient's anatomy, and verified for accurate placement.Once positionally verified, the guide mechanism is fitted with anappropriate guide block 602 comprising a slot 604 for fitting a sawblade 606. The slot 604 is dimensioned to fit the blade, and direct theblade along the preoperatively determined trajectory plane. Shouldadjustments be necessary based on measurements taken by the verificationtool, alternate realignment guide blocks 608 may be used, as shown inFIG. 19b (translational offset) and 19 c (angular offset).

While shown as a slot in a guide block, the guidance tool may provide abearing surface on one or more edges to guide a saw blade or similarimplement (e.g., a chisel). A guidance tool may also comprise anycombination of guide mechanisms to guide a range of medical instruments.For example, a guidance tool may comprise guide mechanisms for setting aplurality of pins, while also providing a guide mechanism suited forreceipt of a guide block dimensioned to guide a saw blade.

The embodiment of the procedure described above enables theintraoperative use of individualized patient-specific templates orguidance tools to increase overall accuracy and success during bone andjoint surgery. As described, the patient-specific guidance tool iscreated from a pre-operative survey of the anatomy of the patient. Thepre-operative steps of surveying and modeling the anatomy provide a planfor central pin alignment based on precision values used duringintraoperative navigation. Furthermore the preoperative planningprovides a mathematical quality value for registration (figure ofmerit), using the individual patient-specific guidance tool. The valuereflects the ability to fit the guidance tool exactly to one position,specific to the patient being treated. In addition, in the event amisalignment is identified intraoperatively, offset drill sleeves can beused to correct the noted misalignment, so as to achieve thepreoperatively defined pin trajectory. In fact, 3-D trajectory analysison 25 hip-resurfacing procedures using the antero-lateral approach hasshown a high degree of trajectory accuracy. The average deviation of thenavigated valgus angle compared to the planned angle was determined tobe 1.46° varus, with a standard deviation of 3.57°. No complicationssuch as femoral neck fracture, sepsis, or inflections have beenobserved.

In summary the preoperative survey and modelling of the patients anatomyand the subsequent planning and production of a patient-specificguidance tool will allow clinics without a “computer-assistedengineering team” in-house to achieve the accuracy of computer-assistedsurgery. As outlined in FIG. 20, the general steps in the overallprocedure are represented as follows:

-   -   a. a suitable medical image (e.g., CT scan) of the patient's        affected anatomy is obtained and a 3-D model is generated (700);    -   b. based on the 3-D model of the patient's anatomy, preoperative        planning is conducted to determine the correct        placement/trajectories of any manipulations (e.g. pin placement,        shaping, etc.) to the anatomy (710);    -   c. based on the results of preoperative planning, the design of        the guidance tool is calculated, taking into consideration        factors such as, but not limited to, the specific registration        surface of the patient's anatomy (720);    -   d. the design of the guidance tool undergoes inspection,        verification and approval by the requesting user (730);    -   e. the design is converted into a usable physical form by way of        rapid prototype manufacture, and sent to the requesting user        (740);    -   f. the guidance tool is sterilized and prepared for use (either        prior to or upon receipt by the user) (750);    -   the guidance tool is used intraoperatively (760).

The procedure described above has a number of notable advantages overthe prior art. First, the procedure combines the accuracy benefits ofCAS with the precision, repeatability, low cost and ease of use benefitsassociated with surgical alignment tools. By optimizing and customizingthe guidance tool for the specific patient being treated, the tool canbe made smaller, thereby reducing the size of the incision duringsurgery. The procedure is easy to conduct and does not require expensivedisposables generally associated with intraoperative computer assistedsurgery methodologies. The procedure reduces ‘line-of-sight’ problemsassociated with opto-electronic computer assisted surgery. That is, inan operating room configured for conventional computer assisted surgery,there is generally a certain amount of interference between thepersonnel present and the equipment in place. For example, either thesurgeons obstruct the “line-of-sight” of the computer-assist equipment,or the computer-assist equipment interferes with the “line-of-sight” ofthe surgeons. The above noted procedure reduces such “line-of-sight”problems as the quantity of cumbersome equipment in the operating roomis reduced. Moreover, no additional surgical exposure is required. Theguidance tool is customized, thereby increasing the precision, anddecreasing complications and operating room time. The guidance tool isdisposable, and inventory management is optimized. The guidance toolalso allows for controlled translational and angular intraoperativeadjustments for pin placement.

As will be appreciated, for the various alternate applicable procedures,specialized fasteners may be necessary as would be evident to oneskilled in the art. In no way are the techniques, tools andmethodologies meant to be limited to hip resurfacing, or any of theexemplary embodiments.

It will be appreciated that, although embodiments have been describedand illustrated in detail, various modifications and changes may bemade. While several embodiments are described above, some of thefeatures described above can be modified, replaced or even omitted. Forexample, the mating surface for registering the guidance tool on thepatient's anatomy may incorporate any number of characteristiclandmarks, from one to a plurality of such features. Although thedrilling sleeve is described as being a surgical grade metal (e.g.,surgical grade stainless steel, titanium, etc.), other suitablematerials such as surgically compatible polymers may be used. Althoughthe guidance tool is manufactured from acrylonitrile butadiene styrene(ABS), other suitable polymer materials and metals may be used. Althoughrapid prototyping is used to create the guidance tool, other methods maybe used to form the necessary tool, such as CNC milling or moldingtechnologies. Although the central pin inserted into the femoral headusing the guidance tool is provided with a self-tapping threadedconfiguration, other suitable pin-anchoring alternatives may beemployed, as would be evident to one skilled in the art. Although theverification tool is shown with sliding rulers, the verification toolcould be configured with a geared calliper-like or other mechanism.Although the guidance tool is generally shown as a unitary structure,the guide block and the body section may be manufactured separately andattached post-production. In addition, it may be feasible to reuse theguide-block portion or the tool while restricting the disposable portionto the customized body section comprising the mating surface. Althoughthe guide block is described as having a locking key for each landmark,multiple landmark locations may correspond to a single locking key,wherein the landmarks lie on the same plane defined by the position ofthe locking key. One or more locking keys may be intraoperativelyadjusted, and the amount of adjustment may be determined by suitablemarkings or a scale such as a protractor-like scale on the verificationtool and/or the guidance tool. Alternatively, the verification tool maybe freed from the one or more locking keys and its position adjusted,the amount of adjustment may be determined by suitable markings or ascale such as a protractor-like scale on the verification tool and/orthe guidance tool. Although the verification tool and drilling sleeveeach separately use the same channel during the described process, theguidance tool, in particular the guide block, may be configured withseparate channels or receptacles for receiving each of these components.Still further alternatives and modifications may occur to those skilledin the art. All such alternatives and modifications are believed to bewithin the scope of the invention and are covered by the claims appendedhereto.

The contents of all cited publications are incorporated herein byreference in their entirety.

1. A preoperatively designed guidance tool for intraoperative use duringbone or joint surgery wherein said guidance tool is specific to theanatomy of the patient being treated, said guidance tool comprising: abody portion; a mating surface provided on said body portion, saidmating surface for positioning said guidance tool on a correspondingregistration surface of a patient's anatomy; and at least one guidemechanism provided on said body portion, said at least one guidemechanism for guiding at least one medical instrument at one or morepreoperatively defined trajectories relative to a patient's anatomy;wherein said at least one guide mechanism is adjustable to alter the oneor more preoperatively defined trajectories if necessary duringintraoperative use.
 2. The guidance tool of claim 1, wherein said bodyportion comprises a guide component, and a registration component, eachof said at least one guide mechanism and said mating surface beingprovided on at least one of said guide component and said registrationcomponent.
 3. The guidance tool of claim 2, wherein said body portionfurther comprises a stability component.
 4. The guidance tool of claim1, wherein said mating surface comprises a profile that is complementaryto characteristic anatomical landmarks found on said registrationsurface of a patient's anatomy.
 5. The guidance tool of claim 1, furthercomprising a removable verification tool for mechanically verifying thecorrect position of the guidance tool on the patient's anatomy, havingregard to preoperatively selected anatomical landmarks.
 6. The guidancetool of claim 5, wherein said verification tool is configured forinsertion into said guide mechanism.
 7. The guidance tool of claim 5,wherein said verification tool comprises at least one graduated sectionto determine misalignment of the guidance tool relative to said one ormore preoperatively defined trajectories.
 8. The guidance tool of claim7, wherein said verification tool comprises two graduated sections forproviding misalignment values relative to the patient's anatomy, a firstgraduated section providing misalignment values in a longitudinaldirection, and a second graduated section providing misalignment valuesin a radial direction.
 9. The guidance tool of claim 5, furthercomprising locking keys provided on said body portion for aligning saidverification tool in a preoperatively defined position for comparisonwith previously selected anatomical landmarks.
 10. The guidance tool ofclaim 1, wherein said guide mechanism comprises a guide channel andcorresponding instrument sleeve for placement within said guide channel,said instrument sleeve for guiding said medical instrument at said oneor more preoperatively defined trajectories relative to a patient'sanatomy.
 11. The guidance tool of claim 10, wherein said guide mechanismcomprises a realignment sleeve for use in place of said instrumentsleeve, said realignment sleeve providing a realignment of theinstrument trajectory where the established trajectory requiresadjustment.
 12. A preoperative process for designing at least oneguidance tool for intraoperative use during bone or joint surgerywherein said at least one guidance tool is specific to the anatomy ofthe patient being treated, said process comprising creating a 3-Dsurface model of the patient's anatomy; using the 3-D surface model ofthe patient's anatomy to preoperatively determine a trajectory of atleast one medical instrument during bone or joint manipulation;designing at least one guidance tool for registering against thepatient's anatomy, said at least one guidance tool providing a guidemechanism and a mating surface, said guide mechanism for guiding said atleast one medical instrument during said bone or joint manipulation atone or more preoperatively defined trajectory relative to a patient'sanatomy, said mating surface providing positional registration of saidat least one guidance tool on the patient's anatomy; and preoperativelyassessing the fit, size and design of the at least one guidance tool onthe patient anatomy using the 3-D surface model of the patient'sanatomy, and adjusting the design of the at least one guidance tool asnecessary to achieve a correct fit.
 13. The preoperative process ofclaim 12, wherein said 3-D surface model of the patient's anatomy iscreated by one or more of CT, MRI, X-ray, and ultrasound.
 14. Thepreoperative process of claim 12, wherein said assessment of fitinvolves a quantitative figure of merit determination based on anexamination of characteristic landmarks on said mating surface relativeto said patient's anatomy.
 15. The preoperative process of claim 12,further comprising the identification of at least one characteristicanatomical landmark to be used in connection with a removableverification tool for mechanically verifying correct position of the atleast one guidance tool on the patient's anatomy, said verification toolbeing configured for attachment to the at least one guidance tool.
 16. Amethod of facilitating bone or joint surgery using a preoperativelydesigned patient-specific guidance tool, said method comprising thesteps of: placing the guidance tool directly upon the patient's anatomyfor which it has been designed, said guidance tool providing a matingsurface and a guide mechanism, said guide mechanism for guiding at leastone medical instrument during bone or joint manipulation at one ore morepreoperatively defined trajectory relative to a patient's anatomy, saidmating surface providing positional registration of said guidance toolon said patient's anatomy; assessing the position of the guidance toolupon the patient's anatomy; assessing the preoperatively defined one ormore trajectory governed by the guide mechanism of the guidance tool;determining whether adjustments to the guide mechanism are necessary forachieving an optimal trajectory of said at least one medical instrument;and adjusting, if necessary, the guide mechanism of said guidance toolto achieve one or more alternate trajectory for said at least onemedical instrument.
 17. The method according to claim 16, wherein averification tool, fitted to said guide mechanism of the guidance tool,is used to assess the position and orientation of the guidance toolhaving regard to the preoperatively defined trajectory.
 18. The guidancetool of claim 17, wherein said verification tool comprises at least onegraduated section to determine misalignment of the guidance toolrelative to said one or more preoperatively defined trajectories. 19.The guidance tool of claim 18, wherein said verification tool comprisestwo graduated sections for providing misalignment values relative to thepatient's anatomy, a first graduated section providing misalignmentvalues in a longitudinal direction, and a second graduated sectionproviding misalignment values in a radial direction.
 20. The methodaccording to claim 17, wherein said guide mechanism comprises a guidechannel and an instrument sleeve, and wherein a correction to realignsaid trajectory is provided by a realignment sleeve used in place ofsaid instrument sleeve to achieve the corrected trajectory.